In the field of oil well rod pumping systems, it is desirable to monitor the strain (or “load”) on the polished rod. In this way, malfunctions in the system can be detected and resolved to protect an oil pump from damage. Various solutions have been proposed for how to measure this force on the polished rod. For example, U.S. Pat. Pub. No. 2010/0020808 to Lawson et al. proposes two separate load cells that are “stacked” from the top of the polished rod and respectively measure an upward and a downward force on the polished rod. In order to measure the total force acting on the polished rod, the load cells encircle the polished rod in a ring shape, and are “stacked” at installation. Installation and maintenance of the load cells is therefore a complex process that requires sequential adding or removing the load cells from the top of the polished rod. Furthermore, designs such as Lawson's require multiple load cells in order to measure the respective upward and downward forces on the polished rod. Because it requires multiple load cells, this design is relatively expensive, and because of its need for the load cells to encircle the polished rod, this design makes installation and maintenance of the load cells a time-consuming process. Additionally, Lawson separates the load cells from the devices that power, process, and wirelessly transmit the measured load data. This further increases the cost and complexity of production and maintenance.
In other art, such as U.S. Pat. No. 7,513,752 to Boone et al., monitoring devices rely on indirect measurements that may roughly correlate to the force on the polished rod. For example, Boone discloses a monitoring device with an external scissor-like attachment hooked around the harness cables, asserting that a measurement of the restoring force tending to return the cables to their non-displaced position is generally proportional to the tension in the cables. However, this sort of measurement may be relatively inaccurate, and the scissor-like attachment of Boone is necessarily external to the monitoring device, thereby being affected by the outdoor elements and all of the maintenance issues associated therewith.
Furthermore, inclinometers are used in rod pumping systems to measure the angle of a walking beam. This measurement is helpful for determining the operating status of a rod pumping system. For example, an inclinometer installed on a beam of a polished rod pumping system may be used to determine the running distance of the polished rod during pumping operation. Based on the polished rod running distance, information on the pump running status can be obtained.
Existing inclinometers are typically powered by an external power supply, which is often an independent power source needing regular replacement and maintenance. U.S. Pat. No. 7,219,723 to Barnes shows an example of an external power supply. Because the power supply is external to the inclinometer, resulting wires are easily damaged due to the environment (e.g., water and heat). Water can also leak into the inclinometer at the places where the external wires are connected to the inclinometer. Furthermore, installation of the inclinometer is difficult due to the separate external power supply and other external components. Attaching such an inclinometer to the walking beam can be awkward, but placement should be precise in order to obtain accurate measurements.